The principal function of transformer oil is to provide electrical insulation and to cool transformer windings by absorbing the heat generated during operation and conducting it to the exterior surfaces. In switch gear and circuit breakers oil also quenches the electric arc. To carry out its functions in the most efficient way, insulating oil must be of good quality and free of contamination. Some electrical and electronic apparatus requires insulating oil of a superior quality in respect to freedom from contamination in order to insure high dielectric strength and extended service life under extreme stresses. Also, cables, extra high voltage transformers, bushings, and capacitors are in this category and require oil of high purity.
The presence of contaminants in the oil can adversely affect the insulating properties of the oil. Contaminants and products of oil deterioration reduce dielectric strength of oil by forming a path of low resistance for electric discharge through the oil. Further, when the contaminants are combustible gases, the potential for explosion exists.
There are many types of contamination, such as water, solid particles of dust, metals, colloidal carbon, products of corrosion, acids, resins, gums, tar, sludge, organic contaminants and solvents, air and gases.
Contamination enters insulating oil in many ways. Some occurs in various phases of oil transfer from the refinery to the point of use. Some contaminants enter through breather vents or during the process of filling or overhaul. Some moisture and contamination can be found even in new transformers as soluble water and air are present in oil. Water in its free phase may be present either dispersed in form of emulsion or settled at the bottom of the reservoir. With synthetic oils, which are heavier than water, water will tend to collect at the surface or near the top of the transformer.
Contaminants can also be introduced through the degradation of the oil under the extreme conditions existing inside the electrical apparatus. This degradation can give rise to combustible gases. Gases which are typically found in transformer oils include ethane, ethylene, acetylene, methane, carbon dioxide, carbon monoxide, hydrogen and oxygen. Protection of electrical apparatus by continuous or frequent purification of insulating oils will eliminate costly breakdowns and assure a safe and dependable operation.
Purification and reconditioning of transformer oil are practiced in the art. As described in the Institute of Electrical and Electronic Engineers Guide for Acceptance and Maintenance of Insulating Oil in Equipment (ANSI/IEEEC57.106-1977), the reconditioning step when contacting the oil with filter media and/or when performing the mechanical separation of oil and water by centrifuging must be accomplished at low temperature to avoid a high level of dissolved water in the oil, since the saturation amount increases rapidly with temperature. The Guide further describes that in reclaiming, the oil is usually treated by contact with Fuller's earth, an adsorbent clay material which is effective in removing the oxidation products and acids.
The above Guide also describes vacuum dehydrators (degasifiers) as an efficient means of reducing water and gas in the oil to a very low value. There is considerable other literature which describes this vacuum treatment step. Typical references to this step are U.S. Pat. No. 3,675,395 (Jul. 11, 1972) by Baranowski and U.S. Pat. No. 3,339,346, (Sep. 5, 1967) by Buchanan, both of which describe an apparatus for heating, filtering/coalescing and vacuum degasifying of transformer oil, and an article by Schenck in January 1983 Transmission and Distribution magazine describing apparatus for heating, filtering and vacuum degasifying of transformer oils. This latter reference discusses use of a cold trap operating at -85.degree. F. to obtain adequate vacuum to sufficiently reduce the water content to meet specifications. This cold trap is refrigerated with a cascade refrigeration unit to attain the very low temperature.
These methods of degasifying transformer oils require costly equipment and the input of energy to heat the oil, to run the vacuum pumps and to maintain the cold trap system to protect the vacuum pumps. As such, these devices are more appropriately considered stationary devices that might reside in a central power generation station or an oil recycling station. For processing oil from transformers which are located in areas remote from such stations, it is extremely useful to have an oil degasifying device which can be easily transported to the transformer location. Of even greater use is a degasifying unit which is of simple design, inexpensive to build and which requires a minimal input of energy to run. The present invention provides such a device and methods for using this device.